A permanent magnet generator/motor generally includes a rotor assembly having a plurality of equally spaced magnet poles of alternating polarity around the outer periphery of the rotor or, in more recent times, a solid structure of samarium cobalt or neodymium-iron-boron. The rotor is rotatable within a stator which generally includes a plurality of windings and magnetic poles of alternating polarity. In a generator mode, rotation of the rotor causes the permanent magnets to pass by the stator poles and coils and thereby induces an electric current to flow in each of the coils. Alternately, if an electric current is passed through the stator coils, the energized coils will cause the rotor to rotate and thus the generator will perform as a motor.
The permanent magnet materials utilized in the permanent magnet rotor do not hold their magnetic properties above a given temperature, that is, they will lose their magnetic properties if they are subjected to a temperature above a certain limit. Neodymium-iron-cobalt permanent magnets, for example, may be permanently demagnetized if subjected to a temperature above 350 degrees Fahrenheit, while samarium-cobalt permanent magnets may be permanently demagnetized if subjected to temperatures above 650 degrees Fahrenheit. Even operating at temperatures close to the above limits can degrade performance of and/or damage the permanent magnet material.
In use, the permanent magnet material is usually enclosed within a sleeve of non-magnetic material. Intimate contact between the non-magnetic material sleeve and the permanent magnet material is achieved by inserting the permanent magnet into the permanent magnet sleeve with a radial interference fit by any number of conventional techniques.
During operation of the permanent magnet generator/motor, the non-magnetic sleeve is subjected to eddy currents and aerodynamic heating. Because of the intimate contact between the non-metallic sleeve and the permanent magnet, the heat from the non-magnetic sleeve is transferred to the permanent magnet. Such heating must be taken into consideration when selecting permanent magnet materials for a permanent magnet generator/motor.
Since the radial air gap between the outer diameter of the permanent magnet rotor sleeve and the inner diameter of the stator is deliberately kept small to enhance magnetic performance, the resulting annular clearance through which cooling air can travel axially severely restricts the flow of cooling air. When this air gap is on the order of 0.050 inches, there simply is not enough annular clearance for sufficient cooling air to flow.
One of the applications of a permanent magnet generator/motor is referred to as a turbogenerator/motor which includes a power head mounted on the same shaft as the permanent magnet generator/motor, and also includes a combustor and recuperator. The turbogenerator/motor power head would normally include a compressor, a turbine and a bearing rotor through which the permanent magnet generator/motor tie rod passes. The compressor is driven by the turbine which receives heated exhaust gases from the combustor supplied with preheated air from the recuperator.